How to Use Oracle Query Tables with Related Tables

As the amount of data in the database increases, the relationships between tables become more and more complex. When we need to query data from some related tables, using Oracle's related query is a very effective method. This article explains how to use Oracle query tables with relational tables.

In Oracle, commonly used correlation query methods include inner join, left join, right join and full outer join.

First, let’s introduce the use of inner connections. An inner join means that only rows with matching data in both tables are returned. For example, if we want to find the intersection data in the customer table (customer) and the order table (order), that is, the customer information of the order in the customer table, we can use the following SQL statement to query:

SELECT a.customer_id, a.customer_name, b.order_id, b.order_date FROM customer a INNER JOIN order b ON a.customer_id = b.customer_id;

The above SQL statement In , the customer table and the order table are related through the INNER JOIN keyword, and the association condition is that the customer_id fields in the two tables are equal. The fields in the SELECT clause are the customer number and customer name in the customer table, and the order number and order date in the order table.

Next, let’s introduce the use of left joins. A left join returns all the rows in the left table and the rows in the right table that match the left table. If there is no matching data in the right table, a NULL value is returned. For example, if we want to find all customer information and their order information, even if they have no orders, we can use the following SQL statement to query:

SELECT a.customer_id, a.customer_name, b.order_id, b.order_date FROM customer a LEFT JOIN order b ON a.customer_id = b.customer_id;

In the above SQL statement, the customer table and order table pass the LEFT JOIN key word association. The fields in the SELECT clause are also the customer number and customer name in the customer table, and the order number and order date in the order table. Note here that since the LEFT JOIN keyword determines to return all rows in the left table, if there is customer information that does not correspond to an order in the customer table, the corresponding order number and order date will be returned as NULL values.

Then, let’s introduce the use of right joins. A right join returns all the rows in the right table and the rows in the left table that match the right table. If there is no matching data in the left table, a NULL value is returned. For example, if we want to find all order information and their customer information, even if there is no customer information corresponding to these orders in the customer table, we can use the following SQL statement to query:

SELECT a.customer_id, a.customer_name, b.order_id, b.order_date FROM customer a RIGHT JOIN order b ON a.customer_id = b.customer_id;

In the above SQL statement, the customer table It is associated with the order table through the RIGHT JOIN keyword. The fields in the SELECT clause are also the customer number and customer name in the customer table, and the order number and order date in the order table. Note here that since the RIGHT JOIN keyword determines to return all rows in the right table, if there is order information that does not correspond to a customer in the order table, the corresponding customer number and customer name will be returned as NULL values.

Finally, let’s introduce the use of full outer joins. Full outer join refers to left and right joins between the left table and the right table and returns all rows. If there is no matching data in the left table, a NULL value is returned; if there is no matching data in the right table, a NULL value is also returned. For example, if we want to find all customer information and their order information, regardless of whether there is matching data in the customer table and order table, we can use the following SQL statement to query:

SELECT a.customer_id, a.customer_name, b.order_id, b.order_date FROM customer a FULL OUTER JOIN order b ON a.customer_id = b.customer_id;

In the above SQL statement, customer The table and order table are related through the FULL OUTER JOIN keyword. The fields in the SELECT clause are also the customer number and customer name in the customer table, and the order number and order date in the order table. Note here that since the FULL OUTER JOIN keyword determines to return all rows in the left and right tables, if there is no corresponding data in the customer table or order table, the corresponding fields will return NULL values.

Through the introduction of this article, I believe you have mastered the method of associated tables in Oracle query tables. In practical applications, choosing the appropriate correlation query method according to needs can improve query efficiency and reduce unnecessary query results.

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